Chronic kidney disease (CKD) with or without uremia affects approximately 10% of worldwide population and increases the risk of developing cardiovascular diseases to several times that of persons with normal renal function because of accelerated atherosclerosis. Multiple studies have shown that atherosclerosis can be initiated by an endothelial cell injury induced by oxidized LDL (oxLDL). Increased levels oxLDL have been recently shown in uremic patients with end-stage renal disease (ESRD), however a mechanistic link between uremia and oxLDL-induced atherosclerosis has not been established. Current theory, developed by Roxborough and Young, suggests that elevated urea results in the formation of carbamylated LDL (cLDL) which makes LDL more susceptible to oxidation. Our data contradict this theory at different levels. In general, our in vitro experiments showed similarities between oxLDL and cLDL action on endothelial and vascular smooth muscle cells, but significantly different levels of cLDL and oxLDL in humans. We hypothesize that blood plasma cLDL is an oxLDL-independent factor that is associated with existing atherosclerotic disease and with its progression in ESRD patients, and is a cause of endothelial cell injury in vitro. A quantitative cLDL sandwich ELISA method recently developed by us showed that concentration of cLDL in human plasma is 5-10 times higher than oxLDL measured by a similar method. Both cLDL and oxLDL are elevated in uremic patients. However, in uremic patients with atherosclerosis measured by intima-media thickness (IMT), cLDL was elevated approximately 200% while oxLDL was not significantly increased compared to uremic patients without thickened IMT. No significant amount of LDL with both types of modifications was found using the ELISA assay of human plasma. Our preliminary data showed that carbamylation of native LDL partially inhibits its oxidation. In vitro, cLDL produced cell injury relevant to atherosclerosis similar to and almost as intense as oxLDL. Changes included endothelial cell death, induction of cell adhesion molecules, attraction of monocytes, and induction of endothelial and vascular smooth muscle cell proliferation. Produced by in vitro chemical modifications, cLDL aggregated more than oxLDL, and cLDL was much more immunogenic in rabbits than oxLDL. Taken together, our data suggest that, at uremic concentrations, cLDL is likely to produce more atherogenic injury to endothelial cells than oxLDL. Our Specific Aims are: (1) To determine whether cLDL is associated with atherosclerotic disease measures, independently of oxLDL, in cross-sectional and prospective studies of ESRD hemodialysis patients;and (2) To quantify and to study mechanisms of plasma cLDL- and oxLDL-mediated endothelial cell injury induced in vitro by normal physiological and uremia-associated concentrations of the LDL isoforms. If according to our expectation, cLDL is shown to be relevant to CKD-associated atherosclerosis, it may be used in the future as a reliable and mechanistically linked cardiovascular risk factor in CKD patients.